1. Field of the Invention
This invention relates to grating lens and a focusing grating coupler which perform focusing by utilizing the diffraction of light.
2. Description of the Related Art
Minimizing the spot size of a focused light beam is important in technology relating to the recording (writing) of data on an optical disk and reproduction (reading) of data from an optical disk. The smaller the spot diameter of the light beam, the higher the recording density can be made.
In order to obtain a small spot size, it is necessary to provide a high numerical aperture (NA) for the objective lens which condenses the light. In a case where the objective lens is a convex lens which performs focusing by utilizing the refraction of the light, a high NA is obtained by increasing the thickness of the convex lens in the direction of the optic axis and shortening the focal distance. With such a convex lens, however, machining is extremely difficult owing to the large curvature of the lens, and therefore the cost of the lens is high. If the objective lens is a micro Fresnel lens which performs focusing by utilizing diffraction of the light, a higher NA is accompanied by a smaller grating period at the periphery of the lens. In actuality, such a lens is difficult to fabricate.
In order to avoid the foregoing problems that accompany an increase in the NA of such lenses, and in order to obtain a smaller diameter for the focused beam spot, a technique is known in which use is made of a super-resolution filter (or coating). For example, refer to Kogaku Gijutsu Contact, Vol. 22, No. 4 (1984), pp. 42-52, and H. Osterberg et al, "The Resolving Power of a Coated Objective", in the Journal of the Optical Society of America, Vol. 39, No. 7, July, 1949, pp. 553-557.
A super-resolution filter described in the former publication is designed to have an amplitude transmittance distribution of the type in which the absorption of light is great at the central portion of the lens and small at the periphery. However, the super-resolution filter for reducing beam spot diameter is itself difficult to fabricate in actual practice, and a laborious operation for achieving alignment between the optic axes of the super-resolution filter and objective lens is required.
Also available in the art is a focusing grating coupler, which is one device expected to be utilizable in a next-generation optical pickup. In a focusing grating coupler, light which propagates along an optical waveguide formed on a substrate is projected to the outside of the substrate surface and focused, whereas entrant light from the outside is introduced to the optical waveguide on the substrate. If such a focusing grating coupler is utilized in an optical pickup, the latter can be made more compact and lighter in weight. For example, see Ura et. al. "Focusing Grating for Integrated Optical-Disk Pickup Device" in The Transactions of IECE of Japan, Part C, Vol. J68-C, No. 10, 1985, October, pp. 803-810, and Ura et. al. "Aberrations of Focusing Grating for Integrated-Optic Disk-Pickup" in the Opto-Quantum Electronics Gijutsu Kenkyu Hokoku OQE86- 84(1986) of the Denshi Joho Tsushin Gakkai (IEICE of Japan).
However, it is required that the focused spot diameter be reduced to about 1 .mu.m in order to utilize the focusing grating coupler in an optical pickup. Consequently, the NA must be made large. In order to obtain a high NA, as mentioned above, the period of the grating must be made very small, the allowable error in terms of precision is small, and fabrication is difficult.